A pulse oximeter clips onto your fingertip and shines two beams of light through your skin to measure how much oxygen your blood is carrying. It works by exploiting a simple fact: blood that’s rich in oxygen and blood that’s low in oxygen absorb light differently. The device reads those differences and displays your oxygen saturation level, typically as a percentage called SpO2, along with your heart rate.
Two Colors of Light, Two Types of Hemoglobin
Inside the clip side of a pulse oximeter are two tiny LEDs. One emits red light and the other emits infrared light (invisible to the eye). On the opposite side of the clip sits a photodetector, a small sensor that measures how much of each light makes it through your finger.
Hemoglobin is the protein in red blood cells that carries oxygen. When hemoglobin is loaded with oxygen, it absorbs more infrared light and lets more red light pass through. When hemoglobin has released its oxygen, the pattern flips: it absorbs more red light and lets more infrared light pass through. The photodetector picks up how much red and infrared light comes through your fingertip, and the device’s processor compares the two signals. That ratio is what determines your oxygen saturation reading.
Why the Pulse Matters
Your finger contains arteries, veins, bone, skin, and other tissue, all of which absorb some light. The device needs to isolate just the arterial blood signal from everything else. It does this by focusing on the pulsatile component of the signal: the tiny, rhythmic expansion of your arteries each time your heart beats. With every heartbeat, a small surge of arterial blood enters the tissue in your fingertip, briefly increasing the amount of light absorbed. Between beats, absorption drops slightly. The pulse oximeter treats anything that pulses and absorbs light as arterial blood, filtering out the steady background absorption from veins, bone, and tissue.
This was the key insight that made modern pulse oximetry possible. Earlier devices required complex calibration and couldn’t separate arterial blood from surrounding tissue. By analyzing only the changing, pulsating portion of the light signal, the device gets a reading that reflects arterial oxygen levels specifically.
How It Calculates Your Heart Rate
Because the device is already tracking the pulsatile signal from your arteries, it gets your heart rate essentially for free. Each pulse of light absorption corresponds to one heartbeat. The processor counts the interval between those pulses and converts it into beats per minute. This is why pulse oximeters display both SpO2 and heart rate on the same screen.
What the Numbers Mean
For most people, a normal SpO2 reading falls between 95% and 100%. A reading of 92% or lower is generally considered concerning enough to contact a healthcare provider. At 88% or below, oxygen levels are dangerously low and warrant emergency care. These thresholds can vary for people with chronic lung conditions, who may have a different baseline that their doctor has discussed with them.
It’s worth understanding the device’s margin of error. The FDA requires pulse oximeters to be accurate within about 2% to 3% of the value you’d get from an arterial blood gas test (the gold standard, where blood is drawn directly from an artery). But that accuracy range holds only about two-thirds of the time. In practical terms, if your oximeter reads 96%, your true saturation could reasonably be anywhere from 93% to 99%. That margin becomes more significant at lower readings, where a few percentage points can mean the difference between adequate oxygen delivery and genuine hypoxia.
What Can Throw Off a Reading
Pulse oximeters depend on light passing cleanly through tissue with a strong arterial pulse. Several things can interfere with that process:
- Nail polish and artificial nails. Dark-colored polish, especially blue, black, or green shades, can absorb light at the same wavelengths the device uses. If you’re getting inconsistent readings, try a different finger or remove the polish.
- Cold hands or poor circulation. The device needs a detectable arterial pulse. Cold fingers, low blood pressure, or peripheral vascular disease can reduce blood flow to the fingertip enough that the oximeter can’t get a reliable signal.
- Movement. Even small finger movements create noise in the light signal. Holding still for 10 to 15 seconds gives the device time to lock onto the pulsatile pattern.
- Bright ambient light. Strong overhead lights or direct sunlight can flood the photodetector and distort the reading. Covering the sensor with your other hand or moving out of direct light can help.
- Tattoos and dried blood. Anything on or in the skin that absorbs light at the relevant wavelengths can skew results.
The Skin Tone Accuracy Gap
Pulse oximeters have a well-documented tendency to overestimate oxygen levels in people with darker skin pigmentation. Because the device relies on light passing through tissue, higher melanin levels change how that light is absorbed, and most devices were historically calibrated on lighter-skinned study populations.
In January 2025, the FDA proposed updated draft guidance specifically aimed at improving pulse oximeter performance across the full range of skin tones. The recommendations call for manufacturers to gather clinical data across diverse skin pigmentations, increase the number of study participants in testing, and use both subjective scales (the Monk Skin Tone Scale) and objective measurements to evaluate device accuracy. Devices that demonstrate comparable performance across skin tones would carry a prominent label stating so. This guidance applies to medical-grade devices, not consumer wellness or fitness products, which aren’t held to the same standards.
If you have darker skin, this doesn’t mean a pulse oximeter is useless. It means the reading may run a few points higher than your true saturation. Being aware of that gap matters most when readings are borderline, in the low-to-mid 90s, where a small overestimate could mask a genuinely low oxygen level.
Fingertip vs. Other Sensor Locations
Fingertip clip-on models are the most common, but pulse oximeters can also be placed on earlobes, toes, or even across the forehead. The principle is identical: light passes through or reflects off tissue with a pulsatile blood supply, and the device reads the absorption difference. Fingertips are preferred because they have a dense capillary bed and are easy to access, but in situations where fingers are too cold or circulation is compromised, an earlobe sensor may pick up a more reliable signal.
What Pulse Oximeters Cannot Detect
A standard pulse oximeter distinguishes between two things: hemoglobin carrying oxygen and hemoglobin not carrying oxygen. It cannot detect carbon monoxide poisoning, because hemoglobin bound to carbon monoxide absorbs light almost identically to oxygen-carrying hemoglobin. In carbon monoxide exposure, an oximeter may display a falsely reassuring reading of 98% or 99% while actual oxygen delivery to tissues is critically low.
The device also can’t tell you anything about how well your lungs are moving carbon dioxide out of your body, or how much oxygen your tissues are actually using. It measures one specific thing, the percentage of hemoglobin in your arterial blood that’s bound to oxygen, and it does that one thing remarkably well for a noninvasive sensor that costs as little as $20.